1. Field of the Invention
The invention relates to the operation of liquid electrolyte fuel cells which use repellancy agents to keep the pores of the gas diffusion electrodes free of electrolyte for the gas access and at the same time provide a large interface for the electrochemical reaction responsible for the production of electricity in the fuel cell.
2. Description of Releated Art
Electrochemical cells invariably comprise at their fundamental level a solid or liquid electrolyte and two electrodes, the anode and cathode, at which the desired electrochemical reactions take place. Porous metal or carbon electrodes are employed in many different electrochemical devices, including metal-air batteries, electrochemical gas sensors, electrosynthesis of useful chemical compounds, and in particular, fuel cells.
A fuel cell is an energy conversion device that efficiently converts the stored chemical energy of its fuel into electrical energy by combining either hydrogen, stored as a gas, or methanol stored as a liquid or gas, with oxygen to generate electrical power. The fuel (e.g. hydrogen) is oxidized at the anode and oxygen (or air) is reduced at the cathode. Both electrodes are of the porous type. The electrolyte has to be in contact with both electrodes and may be acidic or alkaline, liquid, solid or a membrane. The electrodes are designed to be porous and allow the reactant to enter the electrode from the face of the electrode exposed to the reactant fuel supply, and diffuse through the thickness of the electrode to the reaction sites which contain catalysts, usually platinum metal based, to maximize the electrochemical oxidation of hydrogen. The anode is designed be surface-wetted by the electrolyte to contact the same reaction sites. With alkaline electrolyte types the product of the hydrogen reaction is water. The water transpires through the porous electrode into the gas space behind the anode. The cathode is also designed to be porous and allow oxygen or air to enter the electrode and diffuse through to the reaction sites. Catalysts are again commonly incorporated to maximize the rate of the oxygen reaction (peroxide-mechanism) at the cathode reaction sites.
The porous electrodes of fuel cells comprise many components and are typically made up of one or more layers. Typically the gas diffusion electrode will comprise one or more catalyst containing layers, which are supported onto a more rigid porous substrate layer. The catalyst containing layers enhance the desired electrode reactions and comprise a catalyst, which may be formed on a high surface carbon material. Catalysts are often precious metals, particularly platinum alloys in a very high surface area form, dispersed and supported on a high surface area electrically conducting porous carbon, black or graphite. The catalyst component may also be a non precious metal, such as one of the transition metals. In fuel cells which employ alkaline electrolytes, the cathode gas diffusion electrode can comprise catalysts based on macrocyclic compounds of cobalt. The catalyst layers may also comprise a high surface area carbon (steam- or CO2-activated) itself, with no additional metal catalysts. The catalyst layers also comprise other non-catalytic components in addition to the catalyst material, usually polymeric materials which acts as binders to hold the electrode layer together and also performs the additional function of balancing the optimal hydrophobic or hydrophilic nature of the final structure.
These catalyst layers are usually formed into suitable mixtures of the components and deposited into a suitable porous substrate, for example conducting carbon materials such as semi graphitized papers, cloths or foams, or particularly in the case of alkaline electrolyte systems, metal meshes such as nickel or stainless steel. The primary role of the substrate is to act as a physical support for the catalyst containing layers and to provide an electrically conducting structure. It also enables a mechanically stable electrode to be produced.
In view of the above, an improved means of maintaining or regenerating the repellancy properties of fuel cell electrodes is needed.